file/line numbers for doString

[TurkeyMan]

doString is regularly used on fragments of text from containing files. It would be nice if we could supply file/line numbers so that Lua error messages gave useful location information.

As an extension, I often have source files that are composed from many file fragments, and ideally I'd like to be able to populate it with something akin to #line directives, so that error messages track the source files properly throughout the composed string.